Question

13. Only after you put                   to use can you acquire it. (learn)

只有当你将所学的东西应用于实践之后，你才能掌握它。

Answer 1

13. what you have learned考查宾语从句。宾语从句中learn后缺宾语，所以由what引导，根据句意，句子用完成时态。